As the capacity of document data increases, a demand for a document retrieval system for large-capacity document data increases. In order to respond to the increase in the capacity of document data, a retrieval device that divides document data into units suitable for a character string retrieval, sets a character string retrieval index in each unit, narrows down a retrieval target in divided document data units based on a retrieval condition, and retrieves an index in the narrowed document data using a character string included in the retrieval condition is disclosed in PTL 1.